The invention relates to a method of conditioning an oxygen-containing process gas in an electrochemical process in which a gas diffusion electrode, in particular an oxygen-consuming electrode, is used. Here, electrochemical processes are, in particular, chloralkali and hydrochloric acid electrolysis using oxygen-consuming electrodes.
The use of gas diffusion electrodes enables energy savings to be achieved in various electrochemical processes, and in addition the formation of undesirable or uneconomical by-products is avoided.
One example of a gas diffusion electrode is the oxygen-consuming electrode (OCE). Oxygen-consuming electrodes are employed, inter alia, in chloralkali electrolysis, hydrochloric acid electrolysis, fuel cell technology or metal/air batteries.
The invention proceeds from oxygen-consuming electrodes known per se which are configured as gas diffusion electrodes and usually comprise an electrically conductive support and a gas diffusion layer having a catalytically active component.
Various proposals for operating the oxygen-consuming electrodes in electrolysis cells of industrial size are known in principle from the prior art. The basic idea here is to replace the hydrogen-evolving cathode of the electrolysis (for example in chloralkali electrolysis) by an oxygen-consuming electrode (cathode). An overview of possible cell designs and solutions may be found in the publication by Moussallem et al “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
Oxygen-consuming electrodes according to the prior art are used in various arrangements in electrochemical processes, for example in the generation of electric power in fuel cells or in the electrolytic preparation of chlorine from aqueous solutions of sodium chloride. A more detailed description of chloralkali electrolysis using an oxygen-consuming electrode may be found in Journal of Applied Electrochemistry, Vol 38 (9) page 1177-1194 (2008). Examples of electrolysis cells having oxygen-consuming electrodes may be found in the documents EP 1033419B1, DE 19622744C1 and WO 2008006909A2.
The electrolysis of sodium chloride or hydrochloric acid is carried out industrially in plants having capacities of up to over 1 million t of chlorine/annum. The plants encompass not only the electrolysis apparatuses but also facilities for working up chlorine and sodium hydroxide and, if a conventional electrolysis without OCE is operated, hydrogen. Descriptions of the work-up processes may be found, for example, in the sections “Chlorine” and “Sodium Hydroxide” of the on-line edition of Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co KG, Weinheim.
A further development direction for the utilization of OCE technology in chloralkali electrolysis is the ion-exchange membrane which separates the anode space from the cathode space in the electrolysis cell without the sodium hydroxide gap being located directly on the OCE. This arrangement is also referred to as the zero gap arrangement in the prior art. This arrangement is usually also employed in fuel cell technology. A disadvantage here is that the sodium hydroxide formed has to be conveyed through the OCE to the gas side and subsequently flows downward on the OCE. This must not lead to blockage of the pores in the OCE by the sodium hydroxide or to crystallization of sodium hydroxide in the pores. It has been found that very high sodium hydroxide concentrations can also occur, and the ion-exchange membrane is not stable to these high concentrations in the long term (Lipp et al, J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).
A method of recirculating the unconsumed oxygen coming from the electrolysis to the electrolysis is described in DE10149779 A1. In the method described in DE10149779 A1, the fresh oxygen added is depressurized in a gas jet pump and the resulting suction pressure is used for drawing-in the unconsumed oxygen coming from the electrolysis cell. Intimate mixing of fresh oxygen with recycled oxygen occurs in the nozzle.
In principle, a small amount of hydrogen can be formed by means of a secondary reaction in all electrolyses using an OCE and this then leaves the electrolysis cell together with the excess oxygen. On recirculation of the hydrogen-containing oxygen coming from the cell, the hydrogen accumulates and ignitable mixtures can be formed. To avoid a dangerous accumulation of hydrogen and also an undesirable accumulation of other extraneous gases, part of the gas stream leaving the cell is removed as purge stream from the circuit. A further measure to counter dangerous accumulations of hydrogen is the removal by means of catalytic oxidation as described in DE 10342148.
DE10159372 A1 mentions heating and humidification of the process gas as possible variants for an electrochemical half cell having an OCE, but without disclosing further information about the precise temperature conditions, concentrations and appropriate embodiments.
In process technology, heating of process gases is generally effected by means of a heat exchanger which is heated by means of an external energy source such as steam. The temperature of the process gas is controlled by appropriate regulating devices. The regulating devices require additional investment, and the use of an additional external energy source likewise increases the capital costs and also increases the total energy consumption of the process.
It is an object of the present invention to provide a method of heating process gas for use in electrolysis cells having oxygen-consuming electrodes, which method overcomes the above disadvantages.
A specific object of the present invention is to provide a method which allows heating of oxygen-containing feed gas in the electrochemical preparation of chlorine by means of electrolysis apparatuses having OCEs with a minimal outlay in terms of apparatus and instrumentation and without additional energy input.
A particular object of the present invention is to provide a method which allows heating and additionally humidification of oxygen-containing feed gas in the electrochemical preparation of chlorine by means of electrolysis apparatuses having OCEs with a minimal outlay in terms of apparatus and instrumentation and without additional energy input.
The object is achieved by heating the oxygen-containing process gas using heat sources present in the electrolysis process itself or in the subsequent work-up processes.